Chlorosis resistant cytoplasmic male sterile brassica plants

ABSTRACT

The present invention relates to a chlorosis resistant cytoplasmic male sterile (CMS)  Brassica rapa  plant and methods of producing the chlorosis resistant CMS  Brassica rapa  plant. The present invention further relates to progeny, a descendent plant, and/or seed and/or plant part of the chlorosis resistant CMS  Brassica rapa  plant.

The present invention relates to a chlorosis resistant cytoplasmic male sterile (CMS) Brassica rapa plant and methods of producing the chlorosis resistant CMS B. rapa plant. The present invention further relates to progeny, descendent plant, seed or plant part of the chlorosis resistant CMS B. rapa plant.

Cytoplasmic male sterility (CMS) is total or partial male sterility in plants as the result of specific nuclear and mitochondrial interactions. Male sterility is the failure of plants to produce functional anthers, pollen, or male gametes. CMS is under extranuclear genetic control, i.e. under control of the mitochondrial or plastid genomes and is a maternally inherited trait encoded by a gene in the mitochondria and/or plastids. CMS systems are widely exploited in crop plants for hybrid breeding due to the convenience of controlling sterility expression by manipulating the gene-cytoplasm combinations. Male-sterile cytoplasms can be restored to fertility by certain nuclear Rf genes. Several types of CMS can be distinguished on the basis of the specific nuclear genes that restore the failure of plants to produce functional anthers, pollen, or male gametes. Incorporation of CMS systems for male sterility evades the need for emasculation in cross-pollinated species, thus encouraging cross breeding producing only hybrid seeds under natural conditions.

One particular CMS system was developed for use in Brassica plants, especially in Brassica oleracea, B. napus, B. juncea wherein only the cytoplasm (i.e. mitochondria) of Ogura (Japanese radish) was transferred to these Brassica plants. Gene expression changes by dysfunctional mitochondria in Ogura CMS results in pollen development defects, but little is known about gene expression effects in vegetative tissues of the plant. Mitochondrial influence on the nuclear gene expression is referred to as mitochondrial retrograde regulation and it occurs in CMS lines via CMS-inducing genes. In addition, chloroplastic retrograde signalling changes both nuclear and mitochondrial gene expression, and little is known about the regulation of chloroplast genes and nuclear genes for chloroplast proteins by mitochondrial retrograde signalling. However, with the introduction of the CMS also several undesirable characteristics were transferred to the CMS crop which are likely the consequence of incongruity between the Brassica nucleus, chloroplast and Ogura mitochondria.

A drawback of the production of CMS hybrids in Brassica's is that introgression with Ogura cytoplasm into Brassica plants results in chlorosis of the leaves at low temperatures, i.e. discoloration or yellowing of the plant tissues as compared to their fertile analogs. Ogura mitochondria with accompanying B. oleracea chloroplasts in B. rapa cause chlorosis in leaves in colder growing conditions. Recent studies indicate that reduction in thylakoid proteins and chlorophylls lead to reduction in photosynthetic effect and chlorosis of Ogura-CMS at low temperatures. For example, a B. napus plant that was produced by chloroplast fusion and contained the Ogura CMS, showed chlorosis at low temperature (below 15° C.). In Ogura-CMS Chinese cabbage (B. rapa spp pekinensis), showed, next to chlorotic effect, a reduced plant height, delayed flowering, and shorter filaments.

Several attempts have been made to overcome such adverse affect due to the CMS including the chlorosis. Somatic fusion, i.e. chloroplast fusions were attempted by which two distinct species of Brassica plants are fused together to form a new hybrid plant with the characteristics of both. However this approach leads to Brassica lines having severe floral deformities and poor seed setting and chlorosis remained an issue. Others trying to optimize the CMS Ogura-based system observed the production of smaller, less compact, less vigorous, low quality and still chlorotic plant parts and fruits. With the change in cytoplasm genome through addition of chloroplasts of radish with chloroplasts of B. oleracea, some of the negative traits were corrected, however especially chlorosis remained an issue. This because over time, the B. oleracea chloroplasts that are present in the hybrid cells will be outcompeted by the chloroplasts of radish and the chlorosis will remain an issue after several generations of growth. Chloroplasts accompanying Ogura CMS were reported to be preferred during meiosis in mixed chloroplast cytoplasm outnumbering alternative chloroplasts, which resulted in returning of the yellowing phenotype, i.e. chlorosis in later backcross generations. Retrograde signalling or organelle interaction is regulated at the protein level and is unique to the Brassica species used in breeding. Therefore, it seems that finding the optimal combination between nucleus and organelles is a prerequisite for CMS-based breeding.

At present there is no CMS system for hybrid breeding available that overcomes the adverse effects on the plants and fruits, especially over longer periods of time (i.e. several generations of growing and cultivation). Several types of CMS systems are well known and being used in hybrid production, including Polima (pol), hau, nap, and Ogura (ogu) CMS, but all exhibited negative effects on the hybrid plants. The pol CMS is temperature sensitive and therefore limited in its use for hybrid production. Furthermore, the main disadvantage of the pol CMS system is that male sterility is sensitive to the environment in different nuclear backgrounds, leading to its breakdown, and thus (in part) restoration of fertility during the process of hybrid seed production. The negative effect of Ogura cytoplasm on the plants and crops results in commercially less interesting hybrid seed production using such CMS lines. Furthermore, Brassica plants that show chlorosis are not acceptable for commercial sale. However, despite its limitations, the Ogura-CMS system remains the best option (i.e. heaving the least adverse effects) for the production of Brassica hybrid seeds.

Considering the above, there is a need in the art for a CMS Brassica plant that is able to produce hybrid seeds and to resist the negative effects on agronomic performance due to the CMS system, including chlorotic effects at exposure to low temperatures over an extended period of growth and cultivation, a reduced plant height, delayed flowering, and shorter filaments. In addition, there is a need in the art for a method for providing such CMS Brassica plants.

It is an object of the present invention, amongst other objects, to address the above need in the art. The object of present invention, amongst other objects, is met by the present invention as outlined in the appended claims.

Specifically, the above object, amongst other objects, is met, according to a first aspect, by the present invention by a chlorosis resistant cytoplasmic male sterile (CMS) Brassica rapa plant, wherein:

-   -   substantially 100% of the chloroplasts in the plant are Brassica         rapa (B. rapa) chloroplasts, said Brassica rapa chloroplasts are         identifiable by one or more sequences selected from the group         consisting of SEQ ID No. 1, SEQ ID No. 3 and SEQ ID No. 5;     -   the nuclear genome of the plant is a Brassica rapa nuclear         genome identifiable by SEQ ID No. 7 and/or SEQ ID No. 9; and     -   the plant does not comprise Brassica oleracea (B. oleracea)         chloroplasts identifiable by one or more sequences selected from         the group consisting of SEQ ID No. 2, SEQ ID No. 4, and SEQ ID         No. 6.         The cells of the plant comprise chloroplasts and a nucleus that         originate from B. rapa only, wherein the chloroplasts are         associated with SEQ ID No. 1, SEQ ID No. 3 and/or SEQ ID No. 5,         or having at least 95%, preferably at least 96%, more preferably         at least 97%, even more preferably at least 98%, most preferably         at least 99% sequence identity with said sequences. In the CMS         plants of present invention mitochondria of a source providing         CMS (e.g. Ogura) are combined with B. rapa chloroplasts and a B.         rapa nucleus by cell fusion. SEQ ID No. 1, SEQ ID No. 3 and/or         SEQ ID No. 5 identify the origin of the chloroplast organelle         being from B. rapa. The nucleus is associated with SEQ ID No. 7         or 9, or having at least 98%, preferably at least 99%, more         preferably 100% sequence identity with said sequences and         identifies the origin of the nucleus being from B. rapa. The         nucleus of the plant of present invention only comprises the         nucleus of B. rapa and may further be analysed for the absence         of B. oleracea sequences, i.e. by the absence of SEQ ID No. 8 or         SEQ ID No. 10. Further marker analysis may be performed on the         organelles, i.e. mitochondria, nucleus to confirm that the         CMS B. rapa plants comprised 100% of B. rapa chloroplasts,         nucleus and comprise mitochondria of for example Ogura. The B.         rapa plants according to present invention do not show chlorosis         and are suitable for commercial sale. The CMS B. rapa plant of         present invention is similar in size and shape, showing no         adverse affects of the on plant growth and development. Also         after further backcrosses in F1 no chlorosis and adverse effects         on plant growth and plant development was observed in the CMS B.         rapa plants of present invention.

The CMS plant of present invention does not suffer of the consequence of the incompatibility between the Brassica nucleus, chloroplast and Ogura mitochondria, since both the nucleus and chloroplast both fully originate from B. rapa. It is thought that there is no reduction in thylakoid proteins and chlorophylls that will lead to a reduction in photosynthetic efficiency and chlorosis at low temperatures, i.e. no yellowing of the plant parts. There is also no increase in chlorosis after long term breeding, as has been observed in previous CMS Brassica species that have tried to overcome the chlorosis effect. Therefore, a stable chlorosis resistant CMS B. rapa is provided herewith, that shows normal plant growth and no negative effects on agronomic performance. As indicated earlier, regular protoplast fusion in the art is between a B. rapa and a B. oleracea Ogura donor (providing the CMS trait), i.e. using B. oleracea chloroplast and Ogura mitochondrion as donor. This however this leads to Brassica lines showing chlorosis. Surprisingly, the B. rapa of present invention is obtained by protoplast fusion of a selected CMS B. rapa that shows chlorosis (i.e. is not chlorosis resistant) with a wild type B. rapa (non CMS) to provide a new CMS B. rapa plant that is chlorosis resistant and does not suffer from negative effect on plant vigour or seed setting as was shown for other known CMS Ogura-based systems in Brassica.

According to a preferred embodiment, the present invention relates to the chlorosis resistant CMS B. rapa plant, wherein the plant is obtainable by a method comprising the steps of:

-   -   a) protoplast fusion of protoplasts of a fertile Brassica rapa         plant as the protoplast donor with protoplasts of CMS Brassica         rapa plant comprising chloroplasts of another diploid species of         Brassica, preferably Brassica oleracea, as cytoplasm donor;     -   b) selecting protoplast fusion products wherein:         -   substantially 100% of the chloroplasts in the plant are             Brassica rapa chloroplasts identifiable by one or more             sequences selected from the group consisting of SEQ ID No.             1, SEQ ID No. 3 and SEQ ID No. 5;         -   the nuclear genome of the plant is a Brassica rapa nuclear             genome identifiable by SEQ ID No. 7 and/or SEQ ID No. 9; and         -   the plant does not comprise Brassica oleracea chloroplasts             identifiable by one or more sequences selected from the             group consisting of SEQ ID No. 2, SEQ ID No. 4, and SEQ ID             No. 6.

According to a preferred embodiment of the present invention the present plants detailed above are not plants exclusively obtained by means of an essentially biological process. The plant does not comprise chloroplasts that originate from B. oleracea.

According to another preferred embodiment, the present invention relates to the chlorosis resistant CMS B. rapa plant wherein the plant comprises a mitochondrial genome of R. sativus (Radish, also referred to as Ogura or Orgura-CMS), B. oxyrrhina (wild mustard), D. muralis (annual wall-rocket), M. arvensis (wild mint), or E. lyratus (dragonfly), preferably R. sativus. The Ogura-CMS system is the preferred system for providing the mitochondrial genome, i.e. sterility.

According to another preferred embodiment, the present invention relates to the chlorosis resistant CMS B. rapa plant, wherein:

-   -   substantially 100% of the mitochondria are R. sativus         mitochondria identifiable SEQ ID No. 11;     -   the plant does not comprise Brassica oleracea and Brassica rapa         mitochondria.         The cells of the plant comprise a mitochondrial genome         associated with SEQ ID No. 11, or having at least 95%,         preferably at least 96%, more preferably at least 97%, even more         preferably at least 98%, most preferably at least 99% sequence         identity with said sequence and identifies the origin of the         mitochondrial genome being from R. sativus (Ogura). SEQ ID No.         12 to SEQ ID No. 23 are primer sequences that may be used to         determine if the mitochondria comprise B. rapa or B. oleracea         mitochondrial sequences.

According to a preferred embodiment, the present invention relates to the chlorosis resistant CMS B. rapa plant, wherein the B. rapa plant is selected from the group of B. rapa subspecies consisting of rapa, pekinensis, glabra, chinensis, rapifera, oleifera, parachinensis, perviridis, Brassica narinosa, trilocularis, mizuna, preferably rapa or pekenensis. Brassica rapa is a plant consisting of various widely cultivated species including the turnip (Brassica rapa subsp. rapa); napa cabbage (Brassica rapa subsp. pekinensis), bomdong (Brassica rapa var. glabra), bok Choy (Brassica rapa subsp. chinensis), and Rapini (Brassica rapa var. rapifera), an oilseed which has many common names, including turnip rape, field mustard (Brassica rapa subsp. oleifera), bird rape, and keblock. Choy sum (Brassica rapa subsp. parachinensis), Komatsuna (Brassica rapa subsp. perviridis), Tatsoi (Brassica rapa subsp. narinosa), Yellow Sarson (Brassica rapa subsp. trilocularis) and Mizuna (Brassica rapa subsp. niposinica).

According to yet another preferred embodiment, the present invention relates to the chlorosis resistant CMS B. rapa plant, wherein:

-   -   substantially 100% of the chloroplast are chloroplasts derived         from NCIMB Accession Number 43622;     -   substantially 100% of the mitochondria are mitochondria derived         from NCIMB Accession Number 43622.         Representative seeds have been deposited under NCIMB Accession         Number 43622 on 26 May 2020 at NCIMB Ltd. Ferguson Building,         Craibstone Estate, Bucksburn, Aberdeen, AB21 9YA Scotland.

The present invention, according to a further aspect, relates to progeny or a descendent plant, a seed or plant part of the chlorosis resistant CMS B. rapa plant of present invention.

The present invention, according to a further aspect, relates to a hybrid Brassica plant produced using the chlorosis resistant CMS B. rapa plant of present invention. The present invention, according to a further aspect, relates to a method for providing a chlorosis resistant CMS Brassica rapa plant, comprising the steps of;

-   -   a) protoplast fusion of protoplasts of a fertile Brassica rapa         plant as the chloroplast donor with protoplasts of CMS Brassica         rapa plant comprising chloroplasts of another diploid species of         Brassica, preferably Brassica oleracea, as cytoplasm donor;     -   b) selecting protoplast fusion products wherein:         -   substantially 100% of the chloroplasts in the plant are             Brassica rapa chloroplasts identifiable by one or more             sequences selected from the group consisting of SEQ ID No.             1, SEQ ID No. 3 and SEQ ID No. 5;         -   the nuclear genome of the plant is a Brassica rapa nuclear             genome identifiable by SEQ ID No. 7 and/or SEQ ID No. 9; and         -   the plant does not comprise Brassica oleracea chloroplasts             identifiable by one or more sequences selected from the             group consisting of SEQ ID No. 2, SEQ ID No. 4, and SEQ ID             No. 6;         -   optionally, crossing at least one time the selected             chlorosis resistant CMS Brassica rapa plant with a wild type             Brassica rapa plant and selecting a chlorosis resistant CMS             Brassica rapa plant. The method of present invention uses in             step a) the CMS B. rapa that is not chlorosis resistant             (i.e. shows chlorosis) and a wild type B. rapa (non CMS) to             perform a protoplast fusion and to subsequently select             (step c) and provide a new CMS B. rapa plant that is             chlorosis resistant. The chlorosis resistant CMS B. rapa             plant of present invention comprises mitochondria of Ogura             only, and chloroplasts of B. rapa only and a B. rapa             nucleus. Furthermore, this plant does not suffer from             negative effect on plant vigour or seed setting as was shown             for other known CMS Ogura-based systems in Brassica.

According to yet another preferred embodiment, the present invention relates to the method, wherein the mitochondrial genome of the plant is derived from R. sativus (Radish), B. oxyrrhina (wild mustard), D. muralis (annual wall-rocket), M. arvensis (wild mint), E. lyratus (dragonfly), preferably R. sativus.

According to a preferred embodiment, the present invention relates to the method, wherein selecting is further performed by selecting the plants comprising a mitochondrial genome associated with SEQ ID No. 11, and wherein the plant does not comprise B. oleracea or B. rapa mitochondrial sequences. The cells of the plant comprise a mitochondrial genome associated with SEQ ID No. 11, or having at least 95%, preferably at least 96%, more preferably at least 97%, even more preferably at least 98%, most preferably at least 99% sequence identity with said sequence and identifies the origin of the mitochondrial genome being from R. sativus (Ogura). SEQ ID No. 12 to SEQ ID No. 23 are primer sequences that may be used to determine if the mitochondria comprise B. rapa or B. oleracea mitochondrial sequences.

According to yet another preferred embodiment, the present invention relates to the method, wherein the B. rapa plant is selected from the group of B. rapa subspecies consisting of rapa, pekinensis, glabra, chinensis, rapifera, oleifera, parachinensis, perviridis, Brassica narinosa, trilocularis, preferably rapa or pekenensis.

The present invention will be further detailed in the following examples and figures wherein:

FIG. 1 : shows CMS B. rapa plants in a non heated glass house for winter cultivation (temperatures between 5 to 12° C.). The upper 3 to 4 rows of plants comprise the prior art “old” CMS B. rapa plant and show clear yellowing of the leaves, i.e. chlorosis. The lower rows of plants are CMS B. rapa plants according to present invention that is chlorosis resistant, having a B. rapa nucleus, Ogura mitochondria and B. rapa chloroplast.

FIG. 2 : shows leaves of a prior art CMS B. rapa plant (left, CMS3), a CMS B. rapa plant according to present invention having a B. rapa nucleus, Ogura mitochondria and B. rapa chloroplast that is chlorosis resistant (middle, CMS7), and a wild type B. rapa plant (right, WT). The CMS B. rapa plant on the left is showing first signs of chlorosis after 3 to 4 weeks of cultivation in a non heated glass house at temperatures between 5 to 12° C. Chlorosis is especially visible from the stem of the leaves, gradually moving towards the leave. No chlorosis was visible on the CMS B. rapa plant (middle) of present invention during these 3 to 4 weeks under identical conditions, similar to the WT B. rapa. Furthermore, the CMS plant of present invention shows regular growth, comparable to the “old” CMS and wild type B. rapa.

FIG. 3 : shows the flowers of a chlorosis resistant CMS B. rapa plant of present invention (upper picture) and a wild type male fertile B. rapa plant (lower picture). The CMS plant of present invention shows the male sterility throughout the whole plant. Pollen are clearly visible on the wild type male fertile B. rapa plant having fully developed anthers that carry pollen grains, whereas the CMS B. rapa plant of present invention shows undeveloped anthers and the anthers are carrying no pollen at all.

EXAMPLES Generation of Chlorosis Resistant B. rapa Ogura CMS of Present Invention

A B. rapa var. cymosa (Cime di Rapa) fertile line and an Ogura CMS B. rapa (Cime di Rapa) line with Brassica oleracea chloroplasts were grown for 14 days before protoplast fusion. The leaves of a Brassica rapa var. cymosa (Cime di Rapa) fertile line grown for 14 days before protoplast fusion is used and an Ogura CMS Brassica rapa (Cime di Rapa) line with Brassica oleracea chloroplasts grown for 7 days in the dark for etiolated hypocotyl protoplasts are used. Then the leaves of the B. rapa fertile line seedling and the etiolated hypocotyl of the Ogura CMS Brassica rapa (Cime di Rapa) line were used for protoplast fusion known in the art, such as Pelletier et al.1983, Molecular and General Genetics MGG 191:244-250.

Briefly, leaf and hypocotyl material is cut into small pieces and placed in a petri dish containing a layer (˜12 ml) of plasmolysing solution and subsequently wrapped in aluminum foil and stored in a laminar flow cabinet for at least one hour. Next, the plasmolysing solution is replaced by equal amounts of enzyme solution and incubated overnight in aluminum foil at 25° C., wherein the petri dishes comprising the hypocotyl protoplasts is placed on a shaker at 30 rpm and an amplitude of 15 mm Next day, the obtained suspension samples are filtered over a Teflon filter holder with two nylon filters of 110 μm and 53 μm respectively. The filters are re-rinsed with 8-9 ml of CPW16. Then, the suspensions are centrifuged at 110×g for 5 minutes providing visible protoplast bodies. The protoplasts samples are transferred to a fresh centrifuge tube via a Pasteur pipette. Then, approximately 8-9 ml of W5 is added to the protoplast, followed by centrifugation at 75×g for 5 minutes.

Then, the density of the protoplast suspensions is determined with a hemocytometer. The protoplasts of both suspensions protoplasts are brought together with a density of 9×10⁵ protoplasts/ml for fusion in a petri dish as drops using a micro pipette and left to rest in the dark for 15 minutes to enable the protoplasts to adhere to the bottom of the petri dishes. Then approximately 60 μl of PEG1-solution (PEG4000 400 g/l, CaCl₂·2H₂O 7.35 g/l, glucose 54.5 g/l) is added to each drop of protoplasts followed after 3-5 minutes by addition of 4 to 9 ml PEG2 (PEG4000 133 g/l, CaCl₂·2H₂O 9.85 g/l, sorbitol 12.21 g/l, glucose 18.02 g/l) solution. After 3-5 minutes the solution is removed and 4 to 9 ml of PEG3 (PEG4000 67 g/l, CaCl₂·2H₂O 12.2 g/l, sorbitol 15.12 g/l, glucose 9.01 g/l) is added. After 3-5 minutes the solution is removed and 4 to 9 ml of B-medium (according to Pelletier et al., 1983) is added and repeated after 3-5 min for a second time. Finally, the petri dishes are sealed and stored in the dark.

Subsequently, the fusion products being a B. rapa with the nucleus of B. rapa var. cymose, Ogura mitochondria, and B. rapa chloroplasts were grown as described in Pelletier et al. (1983). The fusion products were crossed with a B. rapa line (Cime di Rapa) as BC1. When the BC1 plants were sown and a marker analysis was performed on the three organelles—mitochondria, nucleus and chloroplasts, 100% of the plants confirmed to carry the correct and predicted organelle composition (i.e. nucleus of B. rapa, Ogura mitochondria, and B. rapa chloroplasts). Furthermore, the plants were crossed with a B. rapa nipposinica var. japonica, a parachinensis and a pekinensis type. After 4-5 weeks, seeds were collected and sown for another backcross generation and were also confirmed to carry the correct and predicted organelle composition.

Identification of Chlorosis Resistant B. rapa

A marker analysis was performed on the three organelles—mitochondria, nucleus and chloroplast to confirm that the B. rapa plants comprised 100% of the correct and predicted organelle composition (i.e. B. rapa chloroplasts, nucleus of B. rapa and mitochondria of Ogura). Markers in Table 1 below can be used to confirm that the mitochondria are of Ogura (R. sativus), and that the nucleus and chloroplast are of B. rapa, i.e. and to exclude that the mitochondria are comprised of other sequences not originating from Ogura or to exclude that the chloroplasts are from Brassica sources other than B. rapa, such as chloroplasts that may originate from B. oleracea.

TABLE 1 Marker sequences organelles Brassica plants Sequence Chloroplast marker Cp Rapa 1 (SEQ ID No. 1) GAAATATGACCAACTGTGGTTCGAATATATATAA AAAAAGTTTGTTTTTTTACACTTCTTACTATCAGAT AGTGTGCAT Cp Oleracea 1 (SEQ ID No. 2) GAAATATGACCAACTGTGGTTCGAATATATATAC AAAAAGTTTGTTTTTTTACACTTCTTACTATCAGAT AGTGTGCAT Cp Rapa 2 (SEQ ID No. 3) CTCGAACAAGTAATCGAAAAGATTCTGGAGCATC TTCTGGTTTAGGTATTGCTCCTCCAATGATAGTGG TACCAAGTACTTCTTGGCGAGCTCTAATATGA Cp Oleracea 2 (SEQ ID No. 4) CTCGAACAAGTAATCGAAAAGATTCTGGAGCATC TTCTGGTTTAGGTATTGTTCCTCCAATGATAGTGG TACCAAGTACTTCTTGGCGAGCTCTAATATGA Cp Rapa 3 (SEQ ID No. 5) AAGGATTCATAAGTAGTTGTTAACGGAGTTGAAG TTACCAAATTCTGAGGAGTCGGTATCAATTTATGC CTAATTGCTCCGCCTATAGTTCCCTTAACTACATTT TCTAAACGAGCC Cp Oleracea 3 (SEQ ID No. 6) AAGGATTCATAAGTAGTTGTTAACGGAGTTGAAG TTACCAAATTCTGAGGAGTAGGTATCAATTTATGC CTAATTGCTCCGCCTATAGTTCCCTTAACTACATTT TCTAAACGAGCC Nucleus marker N Rapa 1 (SEQ ID No. 7) TCGGTCAAGGACTCGGATACATCGATGCTGGAGC ACAAAAAGAAGCAGAAGGAAGAGAAAACAGCGA TGGAGGAAGAGATGGAGAAGCTGAGAAGAGATC AGGCGGA N Oleracea 1 (SEQ ID No. 8) TCGGTCAAGGACTCGGATACATCGATGCTGGAGC ATAAGAAGAAGCAGAAGGAAGAGAAAACAGCGA TGGAGGAAGAGATGGAGAAGCTGAGAAGAGATC AGGCGGA N Rapa 2 (SEQ ID No. 9) CTGTGCCTCGGAAAGGTGAGTATCTTGTGTTGGAA GTTTATTTTCTTTGTCTCTCAGTGATCTCATGCATA CACAAACTTTTGTGCAGCCTGCGGATATATACCTG ATTGATGAGCCAAGTG N Oleracea 2 (SEQ ID No. 10) CTGTGCCTCGGAAAGGTGAGTATCTTGTGTTGGAA GTTTGTTTTCTTTGTCTCTCAGTGATCTCATGCATA CACAAACTTTTGTGCAGCCTGCGGATATATACCTG ATTGATGAGCCAAGTG Mitochondria marker Mt Ogura (SEQ ID No. 11) AAGAAGCAAAATCTCATTCAATTTGAAATAGAAG AGATCTCTATGCCCCCTGTTCTTG Mt Rapa 1 Fw (SEQ ID No. 12) TCCCCTCTGTCCCTATGTTG Mt Rapa 1 Rev (SEQ ID No. 13) GAGGTGTTGCCTATCCAGGT Mt Rapa 2 Fw (SEQ ID No. 14) TTCGTTCGTTCACTTCGTTCT Mt Rapa 2 Rev (SEQ ID No. 15) AGGCCTTTCCTTAAGCTTCCT Mt Rapa 3 Fw (SEQ ID No. 16) GGAAGGATCGAACCATAGGAA Mt Rapa 3 Rev (SEQ ID No. 17) TTGATGAGCCTTTACGAGTTGA Mt Oleracea 1 Fw (SEQ ID No. 18) CGAAAACCTTCTGTTCTGTGG Mt Oleracea 1 Rev (SEQ ID No. 19) CGGAGCGTAACCACTTTCTT Mt Oleracea 2 Fw (SEQ ID No. 20) ATTCCCCACCCAACCAATAC Mt Oleracea 2 Rev (SEQ ID No. 21) AAGAGCAGCTTTCTCCGTTCT Mt Oleracea 3 Fw (SEQ ID No. 22) TTGCTGTATCGGAAAGTCCA Mt Oleracea 3 Rev (SEQ ID No. 23) GCATGTCGTAAGCGAGTCAA

For example, the SNP marker SEQ ID No. 1 and SEQ ID No. 2 are used for the identification of the origin of the chloroplast being from B. oleracea or B. rapa. Chloroplasts that originate from the B. oleracea will have a “C” at position 34bp in SEQ ID No. 2, whereas chloroplasts that originate from B. rapa will have an “A” at position 34bp. The SNP marker SEQ ID No. 7 and SEQ ID No.8 are used for the identification of the origin of the genomic DNA being from B. oleracea or B. rapa. When the genomic DNA originates from B. oleracea there will be a “T” at position 36bp and a “G” at position 39bp in SEQ ID No. 8, whereas if the genomic DNA originates from the B. rapa there will be a “C” at position 36bp and an “A” at position 39bp. SEQ ID No. 11 was used for the identification of the origin of the mitochondria being from Ogura to confirm the CMS B. rapa.

In addition, SEQ ID No. 12 to 23 are primer sequences to determine if the mitochondria comprise B. rapa or B. oleracea mitochondrial sequences. PCR reaction was performed on plant total genomic DNA to discriminate between B. rapa and R. sativus mitochondrial genome as well as between B. oleracea and R. sativus mitochondrial genome. Following PCR, the product was incubated with restriction enzyme, according to table 2 and scored for their expected fragment sizes to determine the mitochondrial genome origin. The product after digestion was put on a 2% agarose gel and fragment sizes were analysed. For example, following PCR with SEQ ID No. 14 and 15 and digestion with DdeI, the R. sativum specific mitochondrion DNA product contains 4 fragments of 357, 81, 63 and 57 base pairs in size, whereas the B. rapa specific mitochondrion DNA product contains 5 fragments of 285, 81, 75, 63 and 57 base pairs in size.

TABLE 2 Mitochondrial genome analysis Brassica Primer Product combination size Restriction Products after restriction (nt) per mitochondria (SEQ ID No.) (nt) Enzyme B. rapa B. oleracea R. sativus 12 + 13 1417 Hpy188I 687 + 397 + — 571 + 397 + 281 + 39 + 281 + 117 + 17 39 + 17 14 + 15 549 Dde1 285 + 81 + — 357 + 81 + 75 + 63 + 63 + 57 57 16 + 17 673 HaeIII 377 + 164 + — 377 + 246 + 82 + 50 50 18 + 19 420 Bsp119l — 420 341 + 40 + 40 20 + 21 773 ApoI — 773 474 + 303 22 + 23 568 SacI — 317 + 182 + 317 + 255 77

Four plants have been included for marker analysis; B. rapa (wild type), B. oleracea (wild type), B. rapa “old” CMS (chlorotic), B. rapa “new” CMS (non-chlorotic, plant of present invention). Table 3 provides an overview, wherein “+” indicated that the marker was present and a “−” indicated that the markers were absent in the plants. In respect to the PCR markers (SEQ ID No. 12 to 23) on the basis of the product fragments the plants were scored R (rapa) or O (oleracea), indicating the origin of the mitochondrial genome, and the “−” indicated that the PCR did not yield a PCR product, i.e. an absence of B. rapa or B. oleracea mitochondrial sequences.

TABLE 3 Marker analysis on organelles of Brassica plants. Chloroplast Nucleus Mitochondria Seq ID No. 1 2 3 4 5 6 7 8 9 10 11 12 + 13 14 + 15 16 + 17 18 + 19 20 + 21 22 + 23 B. rapa + − + − + − + − + − − R R R − − − B. oleracea − + − + − + − + − + − − − − O O O B. rapa − + − + − + + − + − + − − − − − − “old” CMS B. rapa + − + − + − + − + − + − − − − − − “new” CMS

Results show that plants of present invention (B. rapa “new” CMS) only comprise chloroplasts and a nucleus that originate from B. rapa, wherein the chloroplasts are 100% associated with markers for B. rapa chloroplasts, and the nucleus is associated with markers for B. rapa nucleus. The chlorosis resistant CMS B. rapa plant of present invention was also tested to comprise only a mitochondrial genome of the Ogura type associated with SEQ ID No. 11, i.e. being a sterile (CMS) plant. No mitochondria genome of B. rapa or B. oleracea was present in the plant of present invention as indicated with the markers SEQ ID No 12 to 23.

Evaluation of Chlorosis in B. rapa

To evaluate yellowing in the developed plant materials, CMS B. rapa plant according to present invention (“new”), a CMS B. rapa plant known in the art (“old”) and a wild type B. rapa plant have been tested for chlorosis under cold conditions. At the same time, the flower quality, plant growth and stability of sterility have been evaluated. Chlorosis was evaluated prior to flowering in a cold greenhouse was done by eye. After four weeks of seedlings of B. rapa growing in a glass house were transferred to a non heated glass house for winter cultivation (at temperatures of between 5 to 12° C.).

Chlorosis was scored by visual inspection of the colour of the plant tissues, which gradually turn from green into yellow as the result of from partial failure to develop chlorophyll. A reduction of chlorophyll leads to yellowing of the tissue. Wild type B. rapa plants that are unaffected (i.e. not affected by a pathogen or nutrient deficiency which causes chlorophyll degradation) are have a normal green phenotype, where no chlorosis was observed and where the plant develops regular, green plant tissue. These plants obtain a score of 5 and are used as the benchmark in a scoring scale ranging from 1 to 5. A score of 1 refers to plants that show severe chlorosis with upper leaves markedly yellow and lower leaves very chlorotic. A score of 2 refers to very chlorotic with pronounced interveinal yellowing. A score of 3 refers to mild to moderate chlorosis, interveinal yellowing on the leaves. A score of 4 refers to near normal (wild type) phenotype, light green/interveinal yellowing, no chlorotic leaves. And a score of 5 thus refers to a green non yellowing phenotype of the B. rapa plant. B. rapa plants that obtain a score of 1 to 4, thus show chlorosis and these plants are not acceptable for commercial sale.

After 3-4 weeks and until commercial maturity in the non heated glass house (at temperatures between 5-12° C.), chlorotic symptoms were visible on the “old” CMS B. rapa plants (plant not according to present invention), scoring a 3 on the chlorosis as defined above. No chlorisos was observed on the “new” CMS B. rapa plants of present invention, which were scored a 5 on the chlorosis and was comparable to the wild type B. rapa plant. FIGS. 1 and 2 show plants and leaves of a CMS B. rapa plant according to present invention (“new”) having a B. rapa nucleus, Ogura mitochondria and B. rapa chloroplast that is chlorosis resistant, a wild type B. rapa plant, and a prior art CMS B. rapa plant (“old”) having a B. rapa nucleus, Ogura mitochondria and B. oleracea chloroplast. Chlorosis is especially visible as interveinal yellowing on upper trifoliates and on the stem of the leaves, gradually moving towards the leave on the “old” B. rapa plants. No chlorosis was visible on the CMS B. rapa plant of present invention during these 3 to 4 weeks under identical conditions. All plants in the three B. rapa groups were similar in size and shape, showing no adverse affects of the on plant growth and development. Also after further backcrosses in F1 no chlorosis and adverse effects on plant growth and plant development was observed in the CMS B. rapa plants of present invention.

Furthermore, the flower quality was assessed and plant sterility was conformed at the initial flowering stage. In the B. rapa plant of present invention, no pollen development and no self-pollination occurrence was observed. This assessment was followed twice a week in order to control newly opened flowers over the different stages of flowering until all flowers were opened. In addition, controlled normal development of petals, stigmas and nectar was observed, apart from the expected abnormal pollen anthers carrying no pollen in comparison with a wild type B. rapa plants (Cima di Rapa), see FIG. 3 . FIG. 3 shows the flowers of a chlorosis resistant CMS B. rapa plant of present invention (upper picture) and a wild type male fertile B. rapa plant (lower picture). Pollen are clearly visible on the wild type plant having fully developed anthers that carry pollen grains, whereas the CMS B. rapa plant of present invention shows undeveloped anthers and the anthers are carrying no pollen at all.

Confirmation of female fertility of the B. rapa plant of present invention was performed by artificial hand pollination of sterile flowers with a wild type B. rapa pollen collected from neighbouring plant in the same greenhouse. Seed setting, ripening and drying were controlled during the season every week from pollination to harvest. 

1. A chlorosis resistant cytoplasmic male sterile (CMS) Brassica rapa plant, wherein: substantially 100% of the chloroplasts in the plant are Brassica rapa chloroplasts, said Brassica rapa chloroplasts are identifiable by one or more sequences selected from the group consisting of SEQ ID No. 1, SEQ ID No. 3 and SEQ ID No. 5; the nuclear genome of the plant is a Brassica rapa nuclear genome identifiable by SEQ ID No. 7 and/or SEQ ID No. 9; and the plant does not comprise Brassica oleracea chloroplasts identifiable by one or more sequences selected from the group consisting of SEQ ID No. 2, SEQ ID No. 4, and SEQ ID No.
 6. 2. Chlorosis resistant CMS Brassica rapa plant according to claim 1, wherein the plant is obtainable by a method comprising the steps of: a) protoplast fusion of protoplasts of a fertile Brassica rapa plant as the protoplast donor with protoplasts of CMS Brassica rapa plant comprising chloroplasts of another diploid species of Brassica, preferably Brassica oleracea, as cytoplasm donor; b) selecting protoplast fusion products wherein: substantially 100% of the chloroplasts in the plant are Brassica rapa chloroplasts identifiable by one or more sequences selected from the group consisting of SEQ ID No. 1, SEQ ID No. 3 and SEQ ID No. 5; the nuclear genome of the plant is a Brassica rapa nuclear genome identifiable by SEQ ID No. 7 and/or SEQ ID No. 9; and the plant does not comprise Brassica oleracea chloroplasts identifiable by one or more sequences selected from the group consisting of SEQ ID No. 2, SEQ ID No. 4, and SEQ ID No.
 6. 3. Chlorosis resistant CMS Brassica rapa plant according to claim for claim 2, wherein the plant comprises mitochondria of Raphanus sativus, Brassica oxyrhina, Diplotaxis muralis, Mentha arvensis, or Enarthrocarpus lyratus, preferably Raphanus sativus.
 4. Chlorosis resistant CMS Brassica rapa plant according to any of the claims 1 to 3, wherein: substantially 100% of the mitochondria are Raphanus sativus mitochondria identifiable SEQ ID No. 11; the plant does not comprise Brassica oleracea and Brassica rapa mitochondria.
 5. Chlorosis resistant CMS Brassica rapa plant according to any one of the claims 1 to 4, wherein the CMS Brassica rapa plant is selected from the group consisting of Brassica rapa subspecies: rapa, pekinensis, glabra, chinensis, rapifera, oleifera, parachinensis, perviridis, Brassica narinosa, trilocularis, mizuna, preferably rapa or pekenensis.
 6. Chlorosis resistant CMS Brassica rapa plant according to any one of the claims 1 to 5 wherein: substantially 100% of the chloroplast are chloroplasts derived from NCIMB Accession Number 43622; substantially 100% of the mitochondria are mitochondria derived from NCIMB Accession Number
 43622. 7. A progeny, descendent plant, seed or plant part of the chlorosis resistant CMS Brassica rapa plant of any one of the claims 1 to
 6. 8. A hybrid Brassica plant produced by using the chlorosis resistant CMS Brassica rapa plant of any one of the claims 1 to 7 as a parent.
 9. Method for providing a chlorosis resistant CMS Brassica rapa plant of any one of the claims 1 to 8, comprising the steps of; a) protoplast fusion of protoplasts of a fertile Brassica rapa plant as the protoplast donor with protoplasts of CMS Brassica rapa plant comprising chloroplasts of another diploid species of Brassica, preferably Brassica oleracea, as cytoplasm donor; b) selecting protoplast fusion products wherein: substantially 100% of the chloroplasts in the plant are Brassica rapa chloroplasts identifiable by one or more sequences selected from the group consisting of SEQ ID No. 1, SEQ ID No. 3 and SEQ ID No. 5; the nuclear genome of the plant is a Brassica rapa nuclear genome identifiable by SEQ ID No. 7 and/or SEQ ID No. 9; and the plant does not comprise Brassica oleracea chloroplasts identifiable by one or more sequences selected from the group consisting of SEQ ID No. 2, SEQ ID No. 4, and SEQ ID No. 6; optionally, crossing at least one time the selected chlorosis resistant CMS Brassica rapa plant with a wild type Brassica rapa plant and selecting a chlorosis resistant CMS Brassica rapa plant. 